Regulatory Properties of the Pyruvate-Dehydrogenase Complex from Escherichia coli. Thiamine Pyrophosphate as an Effector

The pyruvate dehydrogenase complex from Escherichia coli is subject to an allosteric control. Pyruvate shows a homotropic cooperative effect. The extent of the cooperativity increases with decreasing concentrations of thiamine pyrophosphate. This cofactor itself exhibits positive cooperativity in steady-state measurements. The saturation curves both of pyruvate and of thiamine pyrophosphate have a very unusual asymmetric shape. The sigmoidal range is limited to low ligand concentrations and changes to a hyperbolic form far below half saturation. It was demonstraed that allosteric systems with more than eight interacting protomers, which obeys the allosteric model of Monod, J., Wyman, J., and Changeux, J.-P. [J. Mol. Biol. 12 (1965) 88–117], behave in this manner. In the absence of substrate and thiamine pyrophosphate, the pyruvate dehydrogenase exists in an inactive form. At low thiamine pyrophosphate concentrations addition of pyruvate leads, after a lag phase of several minutes, to a partial activation. Within less than one second, complete activation occurs when saturating amounts of thaiamine pyrophosphate are added, irrespective of the presence or absence of pyruvate. The lag period increases with decreasing degree of enzyme dilution. This phenomenon is not due to a dissociation and reassociation of subunits.